Effectiveness of 2023 southern hemisphere influenza vaccines against severe influenza-associated illness: pooled estimates from eight countries using the test-negative design.

Summary

In 12,609 hospitalized SARI patients across eight countries, pooled vaccine effectiveness (VE) of 2023 Southern Hemisphere influenza vaccines against influenza-associated hospitalization was 51.9%, with substantial heterogeneity. Where data allowed, VE against ICU admission was also estimated. Benefits were observed in priority groups, including older adults and those with comorbidities.

Key Findings

• Pooled VE against influenza-associated hospitalization: 51.9% (95% CI 37.2–66.7) across eight countries.
• Analysis included 12,609 hospitalized SARI patients (4,388 test-positive; 8,221 test-negative).
• Network enabled VE estimation for priority groups (children, older adults, comorbidities) and, in some settings, against ICU admission.

Clinical Implications

Sustain and target influenza vaccination for high-risk groups; use pooled VE to inform resource allocation and ICU surge planning during influenza seasons.

Limitations

• Substantial between-country heterogeneity; potential residual confounding inherent to observational designs
• Incomplete ICU VE estimates in some countries due to limited data

Future Directions

Refine age- and risk-specific VE estimates, integrate viral genomic data to assess strain mismatch effects, and evaluate programmatic strategies to improve vaccine uptake in priority populations.